Thigh High Heel Boots w/ Large Thigh Opening

I stupidly sold my first pair long ago and kept these simply because they were new, and I liked the style better. Unfortunately, they do not fit my leg and do not stay up, so they look more floppy than sexy at the top. The material is stiff enough, but when I start walking, things start to sort of collapse. You can see in the full leg picture that the top is starting to fall outward as there is a slit in the top. I'm 5'3". These were purchased at a boutique.